Types moved from ollie/session to ollie/agent:
AgentConfig, AgentConfigPath, AgentsDirs, BuildRuntime, Load, Event,
PersistedAgent, LoadPersistedAgent, RestoreHistory, PromptEnv,
History, Runtime, ErrInterrupted, WatchState.
ollie/session is no longer imported directly in top-level 9p/ files
or mgr/format.go and mgr/sessionfile.go (they only need agent types).
WatchAgent const inlined as string literal.